This invention relates to DC motors, and more particularly, to low cost, self-starting brushless DC motors.
In a DC permanent magnet motor, the initial position of the rotor with respect to the stator is important when the windings are energized to start the rotation of the motor. If the rotor position is such that, when the stator windings are first energized, unlike poles of the permanent magnet rotor and the wound stator are aligned, or very nearly aligned, insufficient torque will be developed to cause the rotor to turn to the next angular position where stator winding energization is advanced. The rotor is thus unable to turn from this locked or "cogged" position, or if the rotor rests near this position very slow starting is the result.
Various means have been proposed to overcome the problem of initially positioning the rotor and stator in an electric motor so that the rotor does not become locked. A common technique used for low cost brushless DC motors in sizes of 10 millihorsepower and less uses a reluctance effect of initially positioning the permanent magnetic rotor for proper starting. Modified stator pole shapes have been used to this end. This technique has been observed to be limited due to very small starting torques resulting. An additional disadvantage of this reluctance offset system is that it creates a negative running torque and therefore tends to derate the operating performance of the motor.
Another means to prevent cogging has been to position a permanent magnet within the stator and offset from the main windings of the motor. When the motor is turned off, the permanent magnet causes the rotor to align in a position offset from the main stator pole. When the motor is energized, the offset between the rotor and the stator pole enables starting rotation. However, this detracts from the operation of the motor because its magnetic effects are present while the motor is running, and are not synchronized with the energization of the main windings.
A need thus exists for a self-starting brushless DC motor in which the rotor and stator cannot lock on starting, and in which the starting provisions do not detract from normal motor operation.